1. Field of the Invention
The present invention generally relates to an apparatus and a method for reducing downhole surge pressure while running a liner into a wellbore. More particularly, the invention relates to an apparatus and a method for reducing surge pressure by opening and closing ports to allow fluid and mud flow to flow within an annulus between the wellbore and a circulation tool.
2. Description of the Related Art
For a long time, the oil-well industry has been aware of the problem created when lowering a liner string at a relatively rapid speed in drilling fluid. This rapid lowering of the liner string results in a corresponding increase or surge in the pressure generated by the drilling fluid below the liner string. A liner string being lowered in to a wellbore can be analogized to a tight fitting plunger being pushed in to a tubular housing. Although there is a small annular clearance between the liner and the wellbore, the fluid bypass rate is limited. The faster the liner is lowered, the more fluid builds up below it due to the limited bypass and this creates an increased pressure or surge below the liner as it is lowered in to the wellbore. Of particular concern is surge related damage due to exposed formation below the liner string.
This surge pressure has been problematic to the oil-well industry in that it has many detrimental effects. Some of these detrimental effects are 1) lost volume of drilling fluid; it is not unheard of to lose 50,000 or more barrels of fluid while running the liner, wherein present costs are $40 to $400 a barrel depending on its mixture, 2) resultant weakening and/or fracturing of the formation when this surge pressure in the borehole exceeds the formation fracture pressure, particularly in highly permeable formations, 3) loss of cement to the formation during the cementing of the liner in the borehole due to the weakened and, possibly, fractured formations which result from the surge pressure on those formations, and 4) differential sticking of the drill string or liner being run into a formation during oil-well operations, that is, when the surge pressure in the borehole is higher than the formation fracture pressure, the loss of drilling fluid to the formation allows the drill string or liner to be pulled against the permeable formation downhole thereby sticking the drill string or liner to the permeable formation.
This surge pressure problem is further exasperated when running tight clearance liners or other apparatus in the existing casing. For example, clearances between a typical liner""s Outer Diameter (O.D.) and a casing""s Inner Diameter (I.D.) are xc2xdxe2x80x3 to xc2xcxe2x80x3. The reduced annular area in these tight clearance liner runs results in correspondingly higher surge pressures and heightened concerns over their resulting detrimental effects.
Typically, surge pressures are minimized by decreasing the running speed of the drill string or liner downhole to maintain the surge pressures at acceptable levels. An acceptable level is a level at least where the drilling fluid pressure, including the surge pressure, is at least less than the formation fracture pressure. The problem with decreasing running speed is that more time is required to complete the liner placement. That is economically disadvantageous in today""s environment where drilling rig rates can be as high as $300,000.00 per day.
U.S. Pat. No. 5,960,881, discloses a downhole surge pressure reduction system to reduce the pressure buildup while running in liners. The surge reduction device disclosed therein is located immediately above the top of the liner. Plugging of the float valve at the lower end of the liner can, render the surge pressure reduction system of the ""881 patent ineffective.
U.S. Pat. No. 2,947,363, proposes a fill-up valve for well strings that includes a movable sleeve in a housing. As taught by the ""363 patent, after a predetermined amount of fluid has been admitted, a ball is dropped on the sleeve and pressure applied to move the sleeve downwardly to misalign the ports to a closed port position. Fingers on the sleeve are stated to interlock with teeth to stop upward movement of the sleeve. While the ball could be moved up the housing by an upward flow of pressurized fluid, the ball cannot be blown or forced downwardly through the sleeve. Therefore, this fill-up valve does not provide full opening for inner drill string work to be accomplished at a depth below the fill-up valve.
U.S. Pat. No. 3,376,935, proposes a well string that is partially filled with fluid during a portion of its descent into a well and, thereafter, selectively closed against the entry of further fluid while descent of the well string continues (""935 patent, col. 1, Ins. 25 to 47). As best shown in FIGS. 3 to 5 of the ""935 patent, a ball seats on a ball seat to move the sleeve downwardly to a closed port position. Upon a predetermined pressure the seat deforms, as shown in FIG. 5, to allow the ball to pivot the flapper valve downwardly and pass out of the housing 3 (""935 patent, col. 6, Ins. 32 to 60). The flapper check valve prevents flow of fluid (e.g. drilling fluid) up through the housing (""935 patent, col. 4, Ins. 60 to 73), whether or not the sleeve is in the open port position (FIG. 3) or the closed port position (FIGS. 2, 4 and 5). Additionally, as best shown in FIGS. 1 and 2, the inside diameter of the sleeve is less than the inside diameter of the drill string or pipe interior, thereby creating a restriction in the string. While this tool allows movement of fluids from the annulus, adjacent the ports of the tool, to flow up the drill string, the surge pressure created by apparatus uses, below the tool, is not alleviated.
U.S. Pat. No. 4,893,678, proposes a multiple-set downhole tool and method of use of the tool. While confirming the oil-well industry desire for xe2x80x9cfull borexe2x80x9d opening in downhole equipment, the ""678 patent proposes the use of a ball to move a sleeve to misalign a port in the sleeve and a passage in the housing. Additionally, while the ball can even be xe2x80x9cblown out,xe2x80x9d the stated purpose of the apparatus in the ""678 patent is to activate a tool, and more particularly, to inflate an elastomeric packer (""678 patent, col. 1, Ins. 20 to 25 and col. 3, In. 14 to col. 4, In. 42), not to reduce surge pressure while running a drill string with a casing liner packer or other apparatus downhole.
A Model xe2x80x9cExe2x80x9d xe2x80x9cHydro-Trip Pressure Subxe2x80x9d No. 799-28, distributed by Baker Oil Tools, a Baker Hughes company of Houston, Tex., is installable on a string below a hydraulically actuated tool, such as a hydrostatic packer to provide a method of applying the tubing pressure required to actuate the tool. To set a hydrostatic packer, a ball is circulated through the tubing and packer to the seat in the xe2x80x9cHydro-Trip Pressure Sub,xe2x80x9d and sufficient tubing pressure is applied to actuate the setting mechanism in the packer. After the packer is set, a pressure increase to approximately 2,500 psi shears screws to allow the ball seat to move down until fingers snap back into a groove. The sub then has a full opening, and the ball passes on down the tubing.
U.S. Pat. No. 5,244,044, proposes a similar catcher sub using a ball to operate pressure operated well tools in the conduit above the catcher sub. However, neither the Baker nor the ""044 tool provides for reduction of surge pressure by diverting fluid flow into the annulus between the drill string and casing.
The present invention relates to a downhole surge pressure reduction system for use in the oil-well industry. Typically, the tool that is the subject of the invention is disposed at an upper end of a string of tubulars or liner to be cemented in a wellbore. Installed below the tool is typically a liner hanger running tool that temporarily holds the liner string in the wellbore prior to cementing.
More specifically, this invention relates to an apparatus and a method for reducing surge pressure while running tubulars into a wellbore. In one embodiment, the invention provides a means of pre-selecting a desired hydrostatic wellbore pressure at which a rupture disc will burst causing wellbore fluid to activate a piston that will seal a number of bypass ports. With the piston activated, the tool is effectively closed, and the circulation tool may proceed with cementing or other needed processes.
Alternatively, the tool may be closed by shearing a breakable plug. Shearing of the breakable plug allows fluid to activate the piston in the same manner as if a rupture disc had burst. Both the rupture disc and the breakable plug, or knock-off plug, are forms of frangible members.
In other embodiments, the tool comprises numerous closure members for sealing the circulation or bypass ports. Particularly, these closure members may consist of a breakable piston sleeve or a sleeve lowered or dropped from the surface. Also required is a closing mechanism that consists of the closure member as well as the equipment required to orient and place the closure member. As envisioned, the tool may be closable by more than one method. Thus, it is one object of this invention to provide a tool capable of reducing pressure surges in a wellbore wherein the tool itself is selectively closable.